Types of Selection for AP Biology
Practice problems for these concepts can be found at: Evolution Review Questions for AP Biology
Natural selection can change the frequencies of alleles in populations through various processes. The most commonly described are the following three:
- Directional selection. This occurs when members of a population at one end of a spectrum are selected against, while those at the other end are selected for. For example, imagine a population of elephants with various sized trunks. In this particular environment, much more food is available in the very tall trees than in the shorter trees. Elephants with what length trunk will survive and reproduce the most successfully? Those with the longest trunks. Those with shorter trunks will be strongly selected against (and those in the middle will also be in the middle in terms of success). Over time we expect to see an increasing percentage of elephants with long trunks (how quickly this change occurs depends on the strength of selection—if all the short-trunked elephants die, we can imagine that the allele frequencies will change very quickly) (See Figure 12.1.)
- Stabilizing selection. This describes selection for the mean of a population for a given allele. A real example of this is human infant birth weight—it is a disadvantage to be really small or really big, and it is best to be somewhere in between. Stabilizing selection has the effect of reducing variation in a population (See Figure 12.1.)
- Disruptive selection. Also known as diversifying selection, this process can be regarded as being the opposite of stabilizing selection. We say that selection is disruptive when individuals at the two extremes of a spectrum of variation do better than the more common forms in the middle. Snail shell color is an example of disruptive selection. Imagine an environment in which snails with very dark shells and those with very light shells are best able to hide from predators. Those with an in-between shell color are gulped up like escargot at a cocktail party, creating the double-hump curve seen in Figure 12.1.
Mike (freshman in college): "Learn these selection types … they make good multiple-choice questions."
These three processes describe the way in which allele frequencies can change as a result of the forces of natural selection. It is also important to remember two other types of selection that complement natural selection: sexual selection and artificial selection.
Sexual selection occurs because individuals differ in mating success. In other words, because not all individuals will have the maximum number of possible offspring, there must be some reason why some individuals have greater reproductive success than others. Think about how this is different from natural selection, which includes both reproduction and survival. Sexual selection is purely about access to mating opportunities.
Sexual selection occurs by two primary processes: within-sex competition and choice. In mammals and many nonmammalian species, females are limited in the number of offspring they can produce in their lifetimes (because of internal gestation), while males are not (because sperm are cheap to produce and few males participate in offspring care). Which sex do you think will compete, and which sex will be choosier? In mammals, males compete and females choose. It makes sense that males have to compete because females are a limiting resource, and it makes sense that females are choosy because they invest a lot in each reproductive effort. This leads to the evolution of characters that are designed for two main functions: (1) as weaponry or other tools for male competition (e.g., large testes for sperm competition) and (2) as traits that increase mating opportunities because females prefer to mate with males who have them (e.g., colorful feathers in many birds).
On what do females base their choices? While you need not become an expert on this matter, it is important to remember that female mate choice for certain characters is not random. One hypothesis for why females choose males with colorful feathers, for example, is that colorful feathers indicate good genes, which is important for a female's offspring. Bright colors are costly, so a male with brightly colored feathers is probably healthy (which may, in turn, indicate an ability to reduce parasite load, for example). We call such sexually selected traits that are the result of female choice honest indicators. Keep in mind that selecting a mate for particular features does not necessarily involve conscious thought, and in most animals never does; the female does not think, "Oh! What nice feathers. He must come from good genes." Rather, females who choose males that display honest indicators have more surviving offspring than do females who don't, and as a result, the "choosing males with colorful feathers" trait increases in the population.
When humans become the agents of natural selection, we describe the process as artificial selection. Instead of allowing individuals to survive and reproduce as they would without human intervention, we may specifically select certain individuals to breed while restraining others from doing so. Artificial selection has resulted in the domestication of a wide range of plant and animal species and the selection of certain traits (e.g., cattle with lean meat, flowers with particular color combinations, dogs with specific kinds of skill).